1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a FAX and a copying machine, and particularly to an image forming apparatus including plural toner image forming devices having a photoconductor and a developing device that develops an electrostatic latent image formed on a surface of the photoconductor to a toner image.
2. Description of the Related Art
Conventionally, in an electrophotographic type color image forming apparatus such as a printer, a FAX and a copying machine, a rotary type image forming apparatus (refer to FIG. 19 of JP-A-2001-347384, for example) that forms toner images of plural colors by switching plural developing devices with respect to one photoconductor and a tandem type image forming apparatus (refer to FIGS. 20 and 21 of JP-A-2001-347384, for example) that includes plural toner image forming devices each having one photoconductor and one developing device corresponding to one color and forms a toner image of each color by each of the toner image forming devices have been known.
The tandem type image forming apparatus is easy to form an image at high speed as compared with the rotary type image forming apparatus, but there is a problem that a size of the image forming apparatus tends to increase as compared with the rotary type image forming apparatus since combination of the photoconductor and the developing device aligns in a horizontal direction or a vertical direction, and there is a problem from a viewpoint of miniaturization of the image forming apparatus.
Also, in recent years, performance (printing speed, productivity) equal to or higher than that of a monochrome (black and white) image forming apparatus has been required with respect to the tandem type color image forming apparatus. It is necessary to rotate a photoconductor at high speed in order to improve productivity of a monochrome image in a color image forming apparatus. When a roll-shaped charging device is used as a charging device for uniformly charging a surface of the photoconductor in this case, there is a possibility that the surface of the photoconductor rotating at high speed cannot be charged sufficiently since charging is performed in only a narrow discharge region in which the charging roll makes contact with the surface of the photoconductor. Therefore, a corona discharger which can cope with high-speed rotation and has a relatively wide electric discharge region is desirably adopted as the charging device. However, in the case of using the corona discharger, a small diameter of the photoconductor (large curvature) increases dissipating charges even when the discharge region of the corona discharger is widened, so that it is necessary to increase the diameter of the photoconductor as compared with the case of using the charging roll and there is a problem of upsizing of the image forming apparatus.
Also, a technique for making a diameter of a photoconductor of black color larger than those of photoconductors of other colors in a tandem type image forming apparatus is described in JP-A-2000-242057. In this reference, a charging device and a developing device are larger than those of other colors since space of the periphery of the photoconductor increases by upsizing the diameter of the photoconductor of black color. Also, it is possible to use the charging device etc. of the same size and structure as those of the charging device etc. of other colors instead of the larger charging device, the developing device, etc. of black color.
In the technique described in JP-A-2000-242057, when the developing device or the charging device is upsized, there are problems that it acts to a disadvantage to miniaturization of the image forming apparatus and also the developing device etc. of black color require components separate from those of the developing device etc. of other colors and cost increases. Also, when the charging devices or the developing devices with the same size and structure are used in the charging devices or the developing devices of all the colors, it is necessary to arrange developing devices in the same position with respect to the photoconductors with different radii (curvatures) since positions of magnetic poles of magnet rolls used in developing rolls of the developing devices are the same. Therefore, arrangement of the photoconductors and the developing devices is limited and, for example, positions of the developing devices are limited at 3 o'clock position of the photoconductors as described in FIG. 2 of JP-A-2000-242057. When the positions of the developing devices are set at 3 o'clock position of the photoconductors, there is a problem that a lateral size of a toner image forming device increases and the image forming apparatus cannot be miniaturized.